Conventionally, various rollable metal sheets are produced by, for example, (i) a continuous casting to prepare a 200 mm-thick cast slab, (ii) heating the slab, (iii) hot rolling, (iv) annealing the hot rolled material, (v) cold rolling, and (vi) an optional heat treatment for working. Recent demands for a reduction in production costs have led to proposals of various methods of eliminating the above-described steps (ii) and (iii) including the single roll and twin roll methods, which comprise continuously feeding a molten metal onto a cooling material having one or two cooling surfaces being transferred and renewed for quench solidification, thereby preparing a thin cast sheet having a thickness of several ten .mu.m to about 10 mm. The above-described single and twin roll methods, etc., provide a rollable metal with a high productivity at a low cost but are plagued by several fundamental problems, and therefore, currently the technology in this field is incomplete, although some products have been put to practical use.
Among the above-mentioned problems, one of the most serious is a loss of the rolling property. In general, contrary to the smooth surface of a hot rolled sheet prepared by the conventional hot rolling process, the surface of a thin cast sheet prepared by the single roll method, twin roll method or the like often has an unevenness of as much as several ten % or more of the sheet thickness, due to rippling, and further, suffers from large variations in the thickness of the sheet in the width direction thereof.
This is a defect inherent to the quench solidifying system and is attributable to a localized difference in the shrinkage accompanying the solidification of the molten metal and a thermal deformation of the roll surface, etc., and can be avoided to some extent by attention to the design and operation of the machinery. This alleviation, however, is limited because, in a rollable metal sheet prepared by a series of production steps wherein a cold rolling or warm rolling step is essential, a ripple or the like on the surface of the metal is squeezed in the rolling direction during the rolling and causes a dimple to be formed in the direction of the sheet body; this is the cause of the occurrence of "scab". The above-described uneven portion also is often a cause of a cracking of a fragile material during cold rolling. In recent years, various methods have been proposed, including a method which comprises casting a soft iron sheet or a stainless steel sheet to a thickness of several ten .mu.m to several mm through a single or twin roll method, and annealing and cold rolling the sheet to prepare a foil strip having a thickness of several ten .mu.m to several hundred .mu.m. Nevertheless, in this method also, the uneven portion present in the thin cast sheet is a cause of contraction or breaking during the cold rolling. As described above, compared to the conventional hot rolling process, the single or twin roll method is an excellent production method having not only the merit of a reduction of costs, such as an elimination of steps and the need for less plant and equipment investment, but also having an advantage in that a cold rolled material having a thickness as small as about several ten .mu.m can be directly prepared. These methods, however, have not been put to practical use, due to problems with the rolling process.
In particular, when the above-described thin cast sheet is used as a starting material for stainless steel sheets, technical problems having an influence on the product quality, such as corrosion resistance, appearance, gloss, polishing property and further, in BA products, streaks and defects called "gold dust", exist on the surface of the steel sheet.
The problem of the surface of the stainless steel sheet has hitherto been solved by conducting a mechanical descaling and pickling after annealing the hot rolled sheet, polishing the whole surface of the coil to remove various defects, and cold-rolling the coil with a large number of passes using a sendzimir mill comprising a multiple roll having a small diameter. A process comprising the steps of annealing, picking, surface polishing, and cold rolling by a small diameter roll is an established technique for preparing a thin strip of stainless steel having a fine surface, and 2D, 2B, and BA products specified in JIS have been produced thereby. The manufacturing techniques for these products are disclosed in detail in Sawatani et al, "Seitetsu Kenkyu (Study in Iron Manufacturing)", N292 (1977), p. 100. Further, to satisfy the need for an elimination of steps, studies have been made into the elimination of the steps of annealing the hot rolled sheet and of surface polishing (see Japanese Examined Patent Publication Nos. 57-38654, 59-46287 and 58-56013). In these studies, however, it was found that the elimination of these steps often has an adverse affect on the surface appearance.
In addition to the above-described prior art, when a product is prepared through the use of the above-described thin cast sheet as a starting material, by the conventional process, compared to the conventional process wherein use is made of a hot rolled sheet as a starting material, the crystal grain of the product becomes large, which leads to serious defects, and thus this step reducing process cannot be put to practical use.
As described above, all of the prior art processes aim at a mirror polish effect on the steep sheet, obtained from the roll, or a change in the dispersed state of a carbide of a steel sheet, and do not always provide a satisfactory solution to the problem. Further, when a thin cast slab, in which recent remarkable developments have been made, is used as a starting material, a problem arises in that a grain pattern is formed in the product if only the prior art method is applied. Therefore, a novel and useful method of solving the above problem is required.
When a unidirectional silicon steel is used as the starting material for the above-described thin cast sheet, a proposal has been made for a technique that will solve a problem of the conventional process for preparing a unidirectional silicon steel sheet, i.e., a problem of a limitation of the silicon content to 4% or less, for example, as disclosed in Japanese Unexamined Patent Publication Nos. 55-69223 and 60-38462, a process for preparing a cold-rollable high silicon steel sheet through the use of a starting material comprising a cast slab having a thickness of not more than several hundred .mu.m prepared by continuously feeding a molten metal containing 4 to 10% by weight of silicon, etc., onto a cooling material having a cooling surface being transferred and renewed to quench the molten metal. A similar method is disclosed in Japanese Unexamined Patent Publication No. 63-11619. This method is applied to a cast slab having a thickness of 0.7 to 2.0 mm.
All of the silicon steel sheets prepared by the above-described improved method exhibit a good mechanical property when the silicon content is high, but are unsatisfactory from the viewpoint of obtaining a high magnetic property, and stably reproducing these properties.
Specifically, the process for preparing an unidirectional silicon steel sheet wherein use is made of a step of preparing a thin cast sheet by quench solidification is advantageous in that it is possible to eliminate the hot rolling step and increase the silicon content, but is unsatisfactory in the attaining of excellent properties and in the reproducibility of the conventional material subjected to the hot rolling. Further, a satisfactory cold rolling property is not attained in a high silicon content material.